Two groups of viruses are being studied: isolates of Rous Sarcoma Virus (RSV) and those containing the myc oncogene (MH2, MC29, OK10, CMII). Studies have revealed that RSV has the capacity to transform erythroid cells both in vitro and in vivo but in a manner distinct from avian erythroblastosis virus (AEV). SInce the erb B and src proteins encoded by AEV and RSV, respectively have been previously shown to share a significant amino acid homology, the present observation suggests that both may also share a common functional homology. With regard to the myc containing viruses, a conditional mutant of MH2 has been isolated which permits infected macrophages to differentiate at the nonpermissive temperature. The mutant was characterized in macrophage and fibroblast transformation assays as well as in vivo studies. By comparing the transformaing properties of the ts mutant and wtMH2 viruses in these different systems, the ts mutation within the MH2 genome could be localized to a region of the myc gene which controls macrophage but not fibroblast transformation. Neither the gag-mil nor the myc proteins encoded by the tsMH2 virus appeared to have incurred any molecular weight changes compared to wtMH2.